EP1259791A2

EP1259791A2 - Method for non-invasive spectrophotometric blood oxygenation monitoring

Info

Publication number: EP1259791A2
Application number: EP01932756A
Authority: EP
Inventors: Paul Benni
Original assignee: CAS Medical Systems Inc
Current assignee: CAS Medical Systems Inc
Priority date: 2000-05-02
Filing date: 2001-04-30
Publication date: 2002-11-27
Anticipated expiration: 2021-04-30
Also published as: US20010047128A1; EP1259791A4; JP2003532107A; WO2001084107A2; US6456862B2; AU2001259258A1; WO2001084107A3; EP1259791B1

Abstract

A method and apparatus for non-invasively determining the blood oxygen saturation level within a subject's tissue is provided that utilizes a near infrared spectrophotometric (NIRS) sensor capable of transmitting a light signal into the tissue of a subject and sensing the light signal once it has passed through the tissue via transmittance or reflectance. The method includes the step of determining attenuation of the light signal as the sum of: (i) attenuation attributable to deoxyhemoglobin; (ii) attenuation attributable to oxyhemoglobin; and (iii) attenuation attributable to light scattering within the subject's tissue. The present method also makes it possible to account for attenuation attributable to fixed or constant light absorbing biological tissue components, and attenuation attributable to variable characteristics of the sensor. By determining differential attenuation as a function of wavelength, the attenuation attributable to tissue light scattering characteristics, fixed light absorbing components, and measuring apparatus characteristics are mathematically cancelled out or minimized relative to the attenuation attributable to deoxyhemoglobin, and attenuation attributable to oxyhemoglobin.

Description

METHOD FOR NON-INVASIVE SPECTROPHOTOMETRIC BLOOD OXYGENATION MONITORING

This application claims the benefit of the filing date of United States Provisional Application No. 60/201 ,359, filed 2 May 2000.

BACKGROUND OF THE INVENTION

1. Technical Field.

[0001] This invention relates to methods for non-invasively determining biological tissue oxygenation in general, and to non-invasive methods utilizing near-infrared spectroscopy (NIRS) techniques in particular.

2. Background Information.

[0002] The molecule that carries the oxygen in the blood is hemoglobin.

Oxygenated hemoglobin is called oxyhemoglobin (HbO₂) and deoxygenated hemoglobin is deoxyhemoglobin (Hb). Total hemoglobin is the summation of the two states of hemoglobin (Total Hb = HbO₂+ Hb), and is proportional to relative blood volume changes, provided that the hematocrit or hemoglobin concentration of the blood is unchanged. The mammalian cardiovascular system consists of a blood pumping mechanism (the heart), a blood transportation system (blood vessels), and

a blood oxygenation system (the lungs). Blood oxygenated by the lungs passes through the heart and is pumped into the arterial vascular system. Under normal conditions, oxygenated arterial blood consists predominately of HbO₂. Large arterial blood vessels branch off into smaller branches called arterioles, which profuse throughout biological tissue. The arterioles branch off into capillaries, the smallest blood vessels. In the capillaries, oxygen carried by hemoglobin is transported to the cells in the tissue, resulting in the release of oxygen molecules (HbO₂ => Hb). Under normal conditions, only a fraction of the HbO₂ molecules give up oxygen to the tissue, depending on the cellular metabolic need. The capillaries then combine together into venuoles, the beginning of the venous circulatory system. Venuoles then combine into larger blood vessels called veins. The veins further combine and return to the heart, and then venous blood is pumped to the lungs. In the lungs, deoxygenated hemoglobin Hb collects oxygen becoming HbO₂ again and the circulatory process is repeated. [0003] Oxygen saturation is defined as:

O₀ saturάtion% = - x 100% (Eqn.1)

² (Hb0₂ + Hb)

In the arterial circulatory system under normal conditions, there is a high proportion of

HbO₂ to Hb, resulting in an arterial oxygen saturation (defined as SaO₂ %) of

95-100%. After delivery of oxygen to tissue via the capillaries, the proportion of HbO₂ to Hb decreases. Therefore, the measured oxygen saturation of venous blood

(defined as SvO₂ %) is lower and may be about 70%.

[0004] One spectrophotometric method, called pulse oximetry, determines arterial oxygen saturation (SaO₂) of peripheral tissue (i.e. finger, ear, nose) by monitoring pulsatile optical attenuation changes of detected light induced by pulsatile arterial blood volume changes in the arteriolar vascular system. The method of pulse oximetry requires pulsatile blood volume changes in order to make a measurement.

Since venous blood is not pulsatile, pulse oximetry cannot provide any information about venous blood.

[0005] Near-infrared spectroscopy (NIRS) is an optical spectrophotometric method of continually monitoring tissue oxygenation that does not require pulsatile blood volume to calculate parameters of clinical value. The NIRS method is based on the principle that light in the near-infrared range (700 to 1 ,000 nm) can pass easily through skin, bone and other tissues where it encounters hemoglobin located mainly within micro-circulation passages; e.g., capillaries, arterioles, and venuoles. Hemoglobin exposed to light in the near infra-red range has specific absorption spectra that varies depending on its oxidation state; i.e., oxyhemoglobin (HbO₂) and deoxyhemoglobin (Hb) each act as a distinct chromophore. By using light sources that transmit near-infrared light at specific different wavelengths, and measuring changes in transmitted or reflected light attenuation, concentration changes of the oxyhemoglobin (HbO₂) and deoxyhemoglobin (Hb) can be monitored. The ability to continually monitor cerebral oxygenation levels is particularly valuable for those patients subject to a condition in which oxygenation levels in the brain may be compromised, leading to brain damage or death.

[0006] The apparatus used in NIRS analysis typically includes a plurality of light sources, one or more light detectors for detecting reflected or transmitted light, and a processor for processing signals that represent the light emanating from the light source and the light detected by the light detector. Light sources such as light emitting diodes (LEDs) or laser diodes that produce light emissions in the wavelength range of 700-1 OOOnm at an intensity below that which would damage the biological tissue being examined are typically used. A photodiode or other light source detector is used to detect light reflected from or passed through the tissue being examined. The processor takes the signals from the light sources and the light detector and analyzes those signals in terms of their intensity and wave properties. [0007] It is known that relative changes of the concentrations of HbO₂ and Hb can be evaluated using apparatus similar to that described above, including a processor programmed to utilize a variant of the Beer-Lambert Law, which accounts for optical attenuation in a highly scattering medium like biological tissue. The modified Beer-Lambert Law can be expressed as:

A_λ = -log(l/I₀) = a_λ * C* d * B_λ + G (Eqn.2) wherein "A_λ" represents the optical attenuation in tissue at a particular wavelength λ (units: optical density or OD); "l₀" represents the incident light intensity (units: W/cm²); "I" represents the detected light intensity; " _λ" represents the wavelength dependent absorption coefficient of the chromophore (units: OD * cm^"1 * μM^"1); "C" represents the concentration of chromophore (units: μM); "d" represents the light source to detector (optode) separation distance (units: cm); "B_λ" represents the wavelength dependent light scattering differential pathlength factor (unitless); and "G" represents light attenuation due to scattering within tissue (units: OD).

[0008] Absolute measurement of chromophore concentration (C) is very difficult because G is unknown or difficult to ascertain. However, over a reasonable measuring period of several hours to days, G can be considered to remain constant, thereby allowing for the measurement of relative changes of chromophore from a zero reference baseline. Thus, if time t-i marks the start of an optical measurement (i.e., a base line) and time t₂ is an arbitrary point in time after t-i , a change in attenuation (ΔA ) between t-i and t₂ can be calculated, and variables G and l₀ will cancel out providing that they remain constant.

[0009] The change in chromophore concentration (ΔC = C(t₂) - C(t-ι)) can be determined from the change in attenuation ΔA, for example using the following equation derived from the Beer-Lambert Law:

ΔA = -log{l_t2 / I_tl)_λ = a_λ * AC * d * B_λ (Eqn.3)

Presently known NIRS algorithms that are designed to calculate the relative change in concentration of more than one chromophore use a multivariate form of Equation 2 or 3. To distinguish between, and to compute relative changes in, oxyhemoglobin (ΔHbO₂) and deoxyhemoglobin (ΔHb), a minimum of two different wavelengths are typically used. The concentration of the HbO₂ and Hb within the examined tissue is determined in μmoles per liter of tissue (μM).

[0010] The above-described NIRS approach to determining oxygen saturation levels is useful, but it is limited in that it only provides information regarding a change in the level of blood oxygen saturation within the tissue. It does not provide a means for determining the total level of blood oxygen saturation within the biological tissue. [0011] At present, information regarding the relative contributions of venous and arterial blood within tissue examined by NIRS is either arbitrarily chosen or is determined by invasive sampling of the blood as a process independent from the NIRS examination. For example, It has been estimated that NIRS examined brain tissue consists of blood comprising from about 60 to 80% venous to about 20 to 40% arterial blood. Blood samples from catheters placed in venous drainage sites such as the internal jugular vein, jugular bulb, or sagittal sinus have been used to evaluate NIRS measurements. It has been estimated in animal studies that NIRS interrogated tissue consists of a mixed vascular bed with a venous-to-arterial ratio of about 2:1 as determined from multiple linear regression analysis of sagittal sinus oxygen saturation (SssO₂) and carotid artery oxygen saturation (SaO₂) in comparison to NIRS measured ΔHb and ΔHbO₂. An expression representing the mixed venous / arterial oxygen saturation (SmvO₂) in NIRS examined tissue is shown by the equation:

Smv0₂ = Kv * Sv0₂ + Ka * Sa0₂ (Eqn.4) where "SvO₂" represents venous oxygen saturation; "SaO₂" represents arterial oxygen saturation; and Kv and Ka are the weighted venous and arterial contributions respectively, with Kv + Ka = 1. The parameters Kv and Ka may have constant values, or they may be a function of SvO₂ and SaO₂. Determined oxygen saturation from the internal jugular vein (SijvO₂), jugular bulb (SjbO₂), or sagittal sinus (SssO₂) can be used to represent SvO₂. Therefore, the value of each term in Equation 4 is empirically determined, typically by discretely sampling or continuously monitoring and subsequently evaluating patient arterial and venous blood from tissue that the NIRS sensor is examining, and using regression analysis to determine the relative contributions of venous and arterial blood independent of the NIRS examination. [0012] What is needed, therefore, is a method for non-invasively determining the level of oxygen saturation within biological tissue that can determine the total oxygen saturation level rather than a change in level; a method that provides calibration means to account for light attenuation due to scattering within tissue (G); and a method that can non-invasively distinguish the contribution of oxygen saturation attributable to venous blood and that which is attributable to arterial blood. DISCLOSURE OF THE INVENTION

[0013] It is, therefore, an object of the present invention to provide a method for non-invasively determining the total level of blood oxygen saturation within biological tissue.

[0014] It is a further object of the present invention to provide a method that provides calibration means to account for light attenuation due to scattering within tissue, light attenuation due to fixed tissue absorbers, and light attenuation due to variability between light measuring apparatuses.

[0015] It is a still further object of the present invention to provide a method that can non-invasively distinguish between the contribution of oxygen saturation attributable to venous blood and that attributable to arterial blood. [0016] According to the present invention, a method and apparatus for non- invasively determining the blood oxygen saturation level within a subject's tissue is provided that utilizes a near infrared spectrophotometric (NIRS) sensor capable of transmitting a light signal into the tissue of a subject and sensing the light signal once it has passed through the tissue via transmittance or reflectance. The method includes the step of determining attenuation of the light signal as the sum of: (i) attenuation attributable to deoxyhemoglobin; (ii) attenuation attributable to oxyhemoglobin; and (iii) attenuation attributable to light scattering within the subject's tissue. The present method also makes it possible to account for attenuation attributable to fixed or constant light absorbing biological tissue components, and attenuation attributable to variable characteristics of the sensor. By determining differential attenuation as a function of wavelength, the attenuation attributable to tissue light scattering characteristics, fixed light absorbing components, and measuring apparatus characteristics are mathematically cancelled out or minimized relative to the attenuation attributable to deoxyhemoglobin, and attenuation attributable to oxyhemoglobin.

[0017] In order to account for the resulting minimized differential attenuation attributable to tissue light scattering characteristics, fixed light absorbing components, and measuring apparatus characteristics, each of the parameters must be measured or calibrated out. Since direct measurement is difficult, calibration to empirically determined data combined with data developed using the NIRS sensor is performed by using regression techniques. The empirically determined data is collected at or about the same time the data is developed with the NIRS sensor. Once the calibration parameters associated with attenuation attributable to tissue light scattering characteristics, fixed light absorbing components, and measuring apparatus characteristics have been determined, the NIRS sensor can be calibrated. [0018] The calibrated sensor can then be used to accurately and non-invasively determine the total oxygen saturation level in the original subject tissue or other subject tissue. In addition, if the separation distance ("d") between the light source to the light detector is known or is determinable, and if the value of "B_λ", which represents the wavelength dependent light scattering differential pathlength factor, is known, then the total amount of concentrations of deoxyhemoglobin (Hb) and oxyhemoglobin (HbO₂) within the examined tissue can be determined using the present method and apparatus.

[0019] The calibrated sensor can be used subsequently to calibrate similar sensors without having to invasively produce a blood sample. Hence, the present method and apparatus enables a non-invasive determination of the blood oxygen saturation level within tissue. For example, an operator can create reference values by sensing a light signal or other reference medium using the calibrated sensor. The operator can then calibrate an uncalibrated sensor by sensing the same light signal or reference medium, and subsequently adjusting the uncalibrated sensor into agreement with the calibrated sensor. Hence, once a reference sensor is created, other similar sensors can be calibrated without the need for invasive procedure. [0020] There are, therefore, several advantages provided by the present method and apparatus. Those advantages include: 1) a practical non-invasive method and apparatus for determining oxygen saturation within tissue that can be used to determine the total blood oxygen saturation within tissue as opposed to a change in blood oxygen saturation; 2) a calibration method that accounts for light attenuation due to scattering within tissue (G), fixed tissue absorbers (F), and measuring apparatus variability (N); and 3) a practical non-invasive method and apparatus for determining oxygen saturation within tissue that can distinguish between the contribution of oxygen saturation attributable to venous blood and that saturation attributable to arterial blood.

[0021] In an alternative embodiment, aspects of the above-described methodology are combined with pulse oximetry techniques to provide a non-invasive method of distinguishing between blood oxygen saturation within tissue that is attributable to venous blood and that which is attributable to arterial blood. Pulse oximetry is used to determine arterial oxygen saturation, and the arterial oxygen saturation is, in turn, used to determine the venous oxygen saturation. [0022] These and other objects, features, and advantages of the present invention method and apparatus will become apparent in light of the detailed description of the invention provided below and the accompanying drawings. The methodology and apparatus described below constitute a preferred embodiment of the underlying invention and do not, therefore, constitute all aspects of the invention that will or may become apparent by one of skill in the art after consideration of the invention disclosed overall herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG.1 is a diagrammatic representation of a NIRS sensor placed on a subject's head.

[0024] FIG.2 is a diagrammatic representation of a NIRS sensor.

[0025] FIG.3 is a diagrammatic view of a NIRS sensor.

[0026] . FIG.4 is a block diagram of the present methodology for calibrating a

NIRS sensor.

[0027] FIG.5 is a graph showing an exemplary plot of absorption coefficient vs. wavelength.

DETAILED DESCRIPTION THE INVENTION

[0028] The present method of and apparatus for non-invasively determining the blood oxygen saturation level within a subject's tissue is provided that utilizes a near infrared spectrophotometric (NIRS) sensor that includes a transducer capable of transmitting a light signal into the tissue of a subject and sensing the light signal once it has passed through the tissue via transmittance or reflectance. The present method and apparatus can be used with a variety of NIRS sensors. The NIRS sensor described below, which is the subject of co-pending U.S. Patent application serial no. 09/434,146 filed 4 November 1999 commonly assigned with the present application, discloses a preferred NIRS sensor. The present method is not limited to use with this preferred NIRS sensor, however.

[0029] Referring to FIGS. 1-5, the preferred NIRS sensor includes a transducer portion 10 and processor portion 12. The transducer portion 10 includes an assembly housing 14 and a connector housing 16. The assembly housing 14, which is a flexible structure that can be attached directly to a subject's body, includes one or more light sources 18 and a light detector 20. A disposable adhesive envelope or pad is used for mounting the assembly housing 14 easily and securely to the subject's skin. Light signals of known but different wavelengths from the light sources 18 emit through a prism assembly 22. The light sources 18 are preferably laser diodes that emit light at a narrow spectral bandwidth at predetermined wavelengths. In one embodiment, the laser diodes are mounted within the connector housing 16. The laser diodes are optically interfaced with a fiber optic light guide to the prism assembly 22 that is disposed within the assembly housing 14. In a second embodiment, the light sources 18 are mounted within the assembly housing 14. A first connector cable 26 connects the assembly housing 14 to the connector housing 16 and a second connector cable 28 connects the connector housing 16 to the processor portion 12. The light detector 20 includes one or more photodiodes. The photodiodes are also operably connected to the processor portion 12 via the first and second connector cables 26,28. The processor portion 12 includes a processor for processing light intensity signals from the light sources 18 and the light detector 20. [0030] The processor utilizes an algorithm that characterizes a change in attenuation as a function of the difference in attenuation between different wavelengths. The present method advantageously accounts for but minimizes the attenuation effects of the scattering variable "G", pathlength B*d, and the absorption "F" due to other components present in biological tissue (i.e. bone, water, skin pigmentation, etc.) that have a relatively flat or very low absorption spectra over the measured wavelength range. In addition, the present method accounts for any offset attenuation "N" due to the characteristics of the sensor that may or may not be wavelength independent. The present method algorithm can be expressed as:

^Axι ^~ Ai = u-A2 = 112 (Eqn. 5) where A_λ1 and A^ are in the form of Equation 6 below which is a modified version of Equation 2 that accounts for attenuation due to "F" and "N":

A_λ = -log(l/I₀)_χ = a_λ * C* d * B_λ + G + F + N (Eqn.6) Substituting Equation 6 into Equation 5 for A_λ and A-^, the terms "F" and "N" within Equation 5 are subtracted out, provided they represent constant light absorption over the measurement wavelengths and provided the same sensor is used to sense the light signal at the various wavelengths. Therefore, in the case where the differential pathlength factor B is wavelength independent, then ΔA_λ12 can be expressed as:

ιi₂ = ^1OS[(⁷ _AI / I _λι ) * {hi I I _λ2 )] - Aa_cλl2cdB + AG_λl2 (Eqn. 7) and rewritten in expanded form:

ΔΛ = Krau - CCma.₂ )[Hb]dB + { _Hb0ιλl - a_mθιkl )[Hb0₂ ]dB + AG_{λl2 (E}qn.8)

Alternatively, the differential pathlength factor "B" may be wavelength dependent. In this case, it is desirable to separate B_λ into two components:

B_λ = B * k_λ (Eqn.9) The parameter B is determined at one specific wavelength and the parameter k_λ represents how B would change at other wavelengths. To continue with the mathematical derivations, it is then desirable to combine the pathlength wavelength dependent parameter k_λ to α_λ:

^aλ' = ^aλ * ^kλ (Eqn-10) The parameter α'_λ represents the absorption coefficient α_λ adjusted by pathlength wavelength dependent parameter k_λ. Incorporation of these modifications into Equation 7 results in the following: Au_. = {^amx ~ a_H' _bλ2)[Hb]dB + [ _m' + AG_λl2 (Eqn.11) where:

represents the attenuation attributable to Hb;

represents the attenuation attributable to HbO₂; and

^ΔGλl2 represents the attenuation attributable to light scattering within tissue (G). [0031] In another alternative case, the light absorption due to the fixed tissue absorbers (F), and sensor variability (N) may not be constant over the measuring wavelengths. In this case, differential attenuation as a function of wavelength would result in the parameters ΔF_λ*|₂ and ΔN_λ12, to be included in Equation 7 or Equation 11. _Λi₂ == iμm' xx ^{~ a}' ₂ )[Hb]dB + [a_H'_bθ2λl - a_m' _{0 λ2} )[Hb0₂ ]dB + AG_λl2 + AF_λl2 + AN_λl2 (Eqn.12)

The parameter ΔN_λ12, does not change in magnitude for a particular NIRS sensor. The parameter ΔF_λ12 , by definition, would be the result of differential attenuation due to components that have a relatively flat or very low absorption spectra over the measured wavelength range, and therefore would be a very small and relatively constant value when compared to the differential attenuation due to hemoglobin. Thus, ΔF_λ12 can be seen as a fixed absorber error correcting parameter in Equation 12. Therefore, these parameters then can be summed together by superposition to become ΔG'_λ12:

^ΔGli₂ = ^ΔGAi₂ + ΔF_A12 + AN_m (Eqn. 13) Incorporation of these modifications into Equation 12 results in the following:

λi2 = MI ^~ + (° ι - am' a_.xi foOi}® + ^ΔGli2 (Eqn.14) [0032] Note that if ΔG_λ12 « G_λ1 and G_λ2, the effect of G is minimized within

Equation 11 , in contrast with the effect of G within Equation 2, at the cost of utilizing one more wavelength to determine Hb and HbO₂. Thus, a minimum of three different wavelengths is needed to determine Hb and HbO₂. Also in the alternative case, ΔG'_λ*i₂ minimizes the effects of light attenuation due to scattering within tissue (G), fixed tissue absorbers (F), and sensor variability (N), at the same cost of utilizing one more wavelength.

[0033] The multivariate form of Equation 11 or 14, after mathematical manipulation, is used to determine HbO₂ and Hb with three different wavelengths:

Rearranging and solving for HbO₂ and Hb, simplifying the Δα' matrix into [Δα' ]

(Eqn.16) and rewritten into:

The parameters A_Hb and A_{H 02} represent the product of the matrices [ΔAJ and [Δα'] and the parameters Ψ_Hb and Ψ_Hbθ2 represent the product of the matrices [ΔG'J and [Δα']^"1 . To determine the level of cerebral blood oxygen saturation (CrSO₂), Equation 17 is rearranged using the form of Equation 1 and is expressed as follows:

Note that the pathlength d*B cancels out in the manipulation from Equation 17 to Equation 18.

[0034] The value for CrSO₂ is initially determined from SmvO₂ using Equation 4 and the empirically determined values for SvO₂ and SaO₂. The empirically determined values for SvO₂ and SaO₂ are based on data developed by discrete sampling or continuous monitoring of the subject's blood performed at or about the same time as the sensing of the tissue with the sensor. The temporal proximity of the NIRS sensing and the development of the empirical data helps assure accuracy. The initial values for Kv and Ka within Equation 4 are clinically reasonable values for the circumstances at hand. The values for Aπ_b02 and A_Hb are determined mathematically using the values for lo_λ and l_λ for each wavelength sensed with the NIRS sensor (e.g., using Equation 2 or 6). The calibration parameters Ψ_Hb and Ψ_Hb02, which account for the effects of light attenuation due to scattering within tissue (G), fixed tissue absorbers (F), and measuring apparatus variability (N), are then determined using Equation 18 and nonlinear regression techniques by correlation to different weighted values of SvO₂ and SaO₂; i.e., different values of Ka and Kv. Statistically acceptable values of Kv and Ka and Ψ _b and Ψn_b02 are converged upon using the non-linear regression techniques. Experimental findings show that after proper selection of Ka and Kv, the calibration parameters Ψ_Hb and Ψπ_b02 are constant within a statistically acceptable margin of error for an individual NIRS sensor used to monitor brain oxygenation on different human subjects. In other words, once the sensor is calibrated it can be used on various human subjects and produce accurate information for each human subject. [0035] In the determination of the CrSO₂ percentage, the photon pathlength

"d^*B" cancels out. If, however, the photon pathlength is known or estimated, then the determination of the total value of Hb and/or HbO₂ is possible. For example, if a value for pathlength "d*B" is input into Equation 17 along with the calibration values Ψ_Hb and Ψ_Hb02> then the total value of Hb and/or HbO₂ can be calculated. The light source to detector separation (optode) distance parameter "d" in the pathlength calculation is a measurable value and can be made constant by setting a fixed distance between light source to detector in the NIRS sensor design. Alternatively, the parameter "d" can be measured once the optodes are placed on the subject by use of calipers, ruler, or other distance measurement means. The pathlength differential factor "B" is more difficult to measure and requires more sophisticated equipment. From a large data set of measured neonatal and adult head differential pathlength factor values, an estimation of the value of "B" can be determined within a statistically acceptable margin of error. Substitution of these predetermined values of "B" into Equation 17 results in the determination of the total values of Hb and HbO₂. [0036] An alternative method of determining total values of Hb and HbO₂ combines Equation 3 and Equation 17 together. The multivariate form of Equation 3 is shown below:

At time t = t-i, the values of ΔHb and ΔHbO₂ are zero. Applying Equation 17, and knowing the calibration values of Ψ _b and Ψ_Hb02 at a predetermined differential pathlength factor "B" and optode separation "d", the total absolute values of Hb and HbO₂ are determined at time t = t-i, which are represented by [Hb] _tι and [HbO₂] _ti respectively. At time t=t₂, the values of ΔHb and ΔHbO₂ are then determined using Equation 19. The total values of Hb and HbO₂ are then determined at time t = t₂ using the following equations:

[Hb] = AHb(t₂) + [Hb]_tl (Eqn.20)

[Hb0₂]_t2 = AHb0₂(t₂) + [Hb0₂]_t (Eqn.21)

Equations 20 and 21 are valid only if all the shared parameters in Equations 17 and 19 are exact. Reduced to practice, the advantage of combining Equations 17 and 19 result in improved signal to noise ratio (SNR) in the calculation of the total values for Hb and HbO₂. Conversely, improved SNR in the calculation of CrSO₂ is also obtained from the following expression:

[HbOΛ

CrS0₂(t₂) = * 100% (Eqn.22)

([Hb02]_{h +}[Hb]_t2 )

[0037] After the calibration parameters Ψ_Hb and Ψπ_b02 are determined using the above-described methodology for an individual NIRS sensor, this particular sensor is said to be calibrated. A calibrated NIRS sensor affords accurate measurement of total tissue oxygen saturation, CrSO₂, by non-invasive means. The calibrated sensor can be used thereafter on any human patient, including adults and neonates. Although the present method is described above in terms of sensing blood oxygenation within cerebral tissue, the present method and apparatus are not limited to cerebral applications and can be used to determine blood oxygenation within tissue found elsewhere within the subject's body.

[0038] According to an additional aspect of the present invention, the above- described method can also be used to establish a calibrated "reference" sensor that can be used to calibrate similar sensors through the use of a phantom sample (also referred to as a "reference sample"). The phantom sample has optical characteristics that are similar to the tissue being examined by the NIRS sensor. The calibrated reference NIRS sensor is used to sense the phantom sample and produce reference values. Similar, but uncalibrated, NIRS sensors can thereafter be calibrated by sensing the same phantom sample and adjusting either the hardware of the uncalibrated sensor or the output of the uncalibrated sensor until the output of the uncalibrated sensor agrees with the reference values produced by the calibrated reference sensor. Therefore, the calibration parameters Ψπ_b and Ψ_Hb02 for the uncalibrated sensor would be determined from the phantom sample. This technique makes it unnecessary to calibrate each new sensor in the manner described above, and thereby provides a relatively quick and cost effective way to calibrate NIRS sensors.

[0039] Besides Hb and HbO₂, other biological constituents of interest (e.g., cytochrome aa₃, etc.) could be determined using the multivariate forms of equations 2, 3, 6 or 7. For each additional constituent to be determined, an additional measuring wavelength will be needed.

[0040] In an alternative embodiment, the above-described methodology can be combined with pulse oximetry techniques to provide an alternative non-invasive method of distinguishing between oxygen saturation attributable to venous blood and that attributable to arterial blood. As demonstrated by Equation 4, SmvO₂ is determined by the ratio of venous oxygen saturation SvO₂ and arterial oxygen saturation SaO₂. A calibrated NIRS sensor affords accurate measurement of total tissue oxygen saturation, CrSO₂, by using regression techniques by correlation to mixed venous oxygen saturation SmvO₂. Therefore, the following expression will result: CrS0₂ = Smv0₂ = Kv* Sv0₂ + Ka * Sa0₂ (Eqn. 23)

Non-invasive pulse oximetry techniques can be used to determine the arterial oxygen saturation (SaO₂) of peripheral tissue (i.e. finger, ear, nose) by monitoring pulsatile optical attenuation changes of detected light induced by pulsatile arterial blood volume changes in the arteriolar vascular system. Arterial blood oxygen saturation determined by pulse oximetry is clinically denoted as SpO₂. If NIRS monitoring and pulse oximetry monitoring are done simultaneously and SpO₂ is set equal to SaO₂ in Equation 23, then venous oxygen saturation can be determined from the following expression:

„ _Λ CrSO_ - (Ka * SpθΛ

Sv0₂ = ²— - ^^ (Eqn. 24)

Kv

For the brain, venous oxygen saturation SvO₂ would be determined from internal jugular vein (SijvO₂), jugular bulb (SjbO₂), or sagittal sinus (SssO₂) and the parameters Ka and Kv would be empirically determined during the calibration of the

NIRS sensor. Under most physiological conditions, SpO₂ is representative of brain arterial oxygen saturation SaO₂. Therefore, depending on which venous saturation parameter was used to calibrate the NIRS sensor, this clinically important parameter

(i.e., Sijv0₂, Sjb0₂, or SssO₂) can be determined by Equation 24 by non-invasive means.

[0041] Since many changes and variations of the disclosed embodiment of the invention may be made without departing from the inventive concept, it is not intended to limit the invention otherwise than as required by the appended claims.

What is claimed is:

Claims

1. A method for non-invasively determining a blood oxygen saturation level within a subject's tissue using a near infrared spectrophotometric sensor, said method comprising the steps of: transmitting a light signal into the subject's tissue at a predetermined first intensity using the sensor, wherein the transmitted light signal includes a first wavelength, a second wavelength, and a third wavelength; sensing a second intensity of the light signal, using the sensor, along the first, second, and third wavelengths after the light signal travels through the subject; wherein the sensor is calibrated using empirical data that relates to the subject's tissue that is sensed by the sensor to account for light signal attenuation resulting from light signal scattering within the subject's tissue; determining an attenuation of the light signal for each of the first, second, and third wavelengths using the predetermined first intensity and the sensed second intensity of the first, second, and third wavelengths; determining a difference in attenuation of the light signal between the first wavelength and the second wavelength, and between the first wavelength and the third wavelength; determining the blood oxygen saturation level within the subject's tissue using the difference in attenuation between the first wavelength and the second wavelength, and the difference in attenuation between the first wavelength and the third wavelength.

2. The method of claim 1 , wherein the sensor is calibrated using equation: SmvO₂ = Kv^*SvO₂ + Ka^*SaO₂.

3. The method of claim 2, wherein the sensor is calibrated by using empirical data to determine a first calibration constant and a second calibration constant.

4. The method of claim 3, wherein the step of determining the blood oxygen saturation level within the subject's tissue utilizes the equation:

where Ψ_Hbθ2 represents the first calibration constant, Ψ^, represents the second calibration constant, Aπ_b02 represents a difference in attenuation of light signal attributable to oxyhemoglobin, and A_Hb represents a difference in attenuation of light signal attributable to deoxyhemoglobin.

5. The method of claim 4, further comprising the steps of: determining a photon pathlength d*B; and determining a concentration of oxyhemoglobin and a concentration of deoxyhemoglobin within the subject's tissue using the first and second calibration constants.

6. The method of claim 5, wherein the concentration of oxyhemoglobin and the concentration of deoxyhemoglobin within the subject's tissue are determined using the equation:

7. The method of claim 6, wherein the step of determining a difference in attenuation of the light signal between the first wavelength and the second wavelength utilizes the equation:

Δ _λl2 = log[(l_λl I Io_λl ) * (I_λ21 Io_λ2 )] = Aa_cλl2cdB + AG '_λXVI

and the step of determining a difference in attenuation of the light signal between the first wavelength and the third wavelength utilizes the equation:

Δ ₁₃ = log[(/_A1 / Io_λl ) * (l_λ31 Io_λ3 )] = Aa_cλl3cdB + AG_λl3.

8. The method of claim 6, wherein the step of determining a difference in attenuation of the light signal between the first wavelength and the second wavelength utilizes the equation:

ΔA_λl2 = (a_H' _bλl - a_H' _bλ2 )[Hb]dB + [a_m' _θϊλγ - α^_A2 )[Hb0₂ ]dB + AG_λl2

Δ _λl3 = (a_H' _bλl - a_H' _bλ3 )[Hb]dB + (α^_A1 - α^_A3 )[HbQ₂ B + AG_λl3.

9. The method of claim 6, wherein the step of determining a difference in attenuation of the light signal between the first wavelength and the second wavelength utilizes the equation:

m = te»_λi " m' x₂ )[Hb]dB + (a_H' _bθ2λl - a_m' _θιλ2 )[Hb0₂ ]dB + AG_λ'_l

u = ( m' xι ~ _bχ₃ )[Hb B + (a_H' _bθ2λl - _H' _bθ2λ3 )[Hb0₂ }IB + AG_λ'_l3.

10. The method of claim 1 , wherein the step of determining a difference in attenuation of the light signal between the first wavelength and the second wavelength utilizes the equation:

^₁₂ = ^log[(⁷ i U _λl) * (I_λ21 Io_λ2 )] = Aa_cλl2cdB + AG_λl2 and the step of determining a difference in attenuation of the light signal between the first wavelength and the third wavelength utilizes the equation:

₁₁₃ = ^los[(Λ_ι / I _λl ) * (l_λ31 Io_λ3 )] = Aa_cλl3cdB + ΔG_λl3.

11. The method of claim 1 , wherein the step of determining a difference in attenuation of the light signal between the first wavelength and the second wavelength utilizes the equation:

n = («W " ^am' x₂ )[Hb lB + {a_H' _bθ2λ - a_m' _o%λ )[Hb0₂ ]dB + AG_λl2

ΔAis = {^amxι ^{~ a}m' x₃ )[Hb lB + (a_H' _bθ2λl - a_H' _bθ2λ3 )[Hb0₂ }IB + AG_λl3.

12. The method of claim 1, wherein the step of determining a difference in attenuation of the light signal between the first wavelength and the second wavelength utilizes the equation:

Hi₂ = fe_λi - ^aΗ_bX2 )[Hb iB + ( _H' _bθ2λl - a_H' _b0ϊλ2 )[Hb0₂ ]dB + AG_λ'_l2

Δ is = {^amxι ^{~ a}' x₃ )[Hb]dB + (a_H' _bθ2λl - α^_A3 )[Hb0₂ ]dB + ΔG_λl3.

13. The method of claim 2, further comprising the steps of: determining a blood oxygen saturation level attributable to arterial blood within the subject's tissue using a pulse oximetry technique; and determining a blood oxygen saturation level attributable to venous blood within the subject's tissue using the equation:

CrS0₂ - (Ka * SpQ₂)

SvO, 2 =

Kv

14. A method for determining a blood oxygen saturation level within a subject's tissue using a near infrared spectrophotometric sensor attached to the skin of the subject, said method comprising the steps of: transmitting a light signal into the subject's tissue at a predetermined first intensity, wherein the transmitted light signal includes a first wavelength, a second wavelength, and a third wavelength; sensing a second intensity of the light signal along the first, second, and third wavelengths after the light signal travels through the subject; determining an attenuation of the light signal for each of the first, second, and third wavelengths using the predetermined first intensity and the sensed second intensity of the first, second, and third wavelengths; determining a difference in attenuation of the light signal between the first wavelength and the second wavelength, and between the first wavelength and the third wavelength; determining a first calibration constant and a second calibration constant using empirical data developed from the subject at or about the same time as when the sensing occurs; determining the blood oxygen saturation level within the subject's tissue using the difference in attenuation between the first wavelength and the second wavelength, and the difference in attenuation between the first wavelength and the third wavelength, and the first calibration constant and the second calibration constant.

15. The method of claim 14 wherein the empirical data is collected by discretely sampling a venous blood source and an arterial blood source from the subject.

16. The method of claim 14 wherein the empirical data is collected by continuously monitoring a venous blood source and an arterial blood source from the subject.

17. The method of claim 14, wherein the sensor is calibrated using equation: SmvO₂ = Kv^*SvO₂ + Ka*SaO₂.

18. The method of claim 17, wherein the step of determining the blood oxygen saturation level within the subject's tissue utilizes the equation:

CrS0₂% 100%

where Ψ_Hbθ2 represents the first calibration constant, Ψ _b represents the second calibration constant, A_Hb02 represents a difference in attenuation of light signal attributable to oxyhemoglobin, and Aπ_b represents a difference in attenuation of light signal attributable to deoxyhemoglobin..

19. The method of claim 18, further comprising the steps of: determining a photon pathlength d*B; and determining the concentration of oxyhemoglobin and deoxyhemoglobin within the subject's tissue using the first and second calibration constants.

20. The method of claim 19, wherein the concentration of oxyhemoglobin and deoxyhemoglobin within the subject's tissue are determined using the equation

21. The method of claim 20, wherein the step of determining a difference in attenuation of the light signal between the first wavelength and the second wavelength utilizes the equation:

ΔΛιi₂ = ^g[(l_λl I Io_λl ) * (l_λ21 Io_λ2 )] = Aa_cλl2cdB + AG '_λXVI

ΔΛllS = ^l0g[(⁷ l ' ^IθM ) * (⁷ 3 / ^IθXS )] = ^CCcX13^cdB + ^ΔGA13 ^■

22. The method of claim 20, wherein the step of determining a difference in attenuation of the light signal between the first wavelength and the second wavelength utilizes the equation:

fci + ( _m' _θτλx - a_H' _bθ2λ2 )[Hb0₂ ]dB + AG_λl2

^xn = [μm' x ~ ^a' x₃ )[Hb]ϊB + (α^_A1 - a_H'_b0ιλ3 )[Hb0₂ B + AG_λl3.

23. The method of claim 20, wherein the step of determining a difference in attenuation of the light signal between the first wavelength and the second wavelength utilizes the equation:

- _H' _bθ2λ2 )[Hb0₂ ]dB + AG_λ'_l

and the step of determining a difference in attenuation of the light signal between the first wavelength and the third wavelength utilizes the equation: ιi₃ = (P' xi - + AG_λ ^f _l .

24. The method of claim 14, wherein the step of determining the blood oxygen saturation level within the subject's tissue utilizes the equation:

CrS0₂% = * 100%

where Ψ_Hb02 represents the first calibration constant, Ψ^_b represents the second calibration constant, A_Hb02 represents a difference in attenuation of light signal attributable to oxyhemoglobin, and A_Hb represents a difference in attenuation of light signal attributable to deoxyhemoglobin.

25. The method of claim 24, further comprising the steps of: determining a photon pathlength d*B; and determining the concentration of oxyhemoglobin and deoxyhemoglobin within the subject's tissue using the first and second calibration constants.

26. The method of claim 25, wherein the concentration of oxyhemoglobin and deoxyhemoglobin within the subject's tissue are determined using the equation

27. The method of claim 26, wherein the step of determining a difference in attenuation of the light signal between the first wavelength and the second wavelength utilizes the equation:

ΔA_X12 = log[(/_λl IIo_λl)* (l_λ21 Io_λ2 )] = Aa_cλl2cdB + AG '_λXVI

and the step of determining a difference in attenuation of the light signal- between the first wavelength and the third wavelength utilizes the equation:

ll3 = ^lQg[(Λl ' ^Iθ l ) * (⁷ 3 / ^IθX3 )] = ^ACCc 13^CdB + ΔG ₁₃.

28. The method of claim 26, wherein the step of determining a difference in attenuation of the light signal between the first wavelength and the second wavelength utilizes the equation: m = K_MI " W_{Hb 2} )[Hb]dB + [a_H' _bθ2λx - a_H' _bθ2λ2 )[Hb0₂ ]dB + ΔG_A12

m = ( ' xx ~ « i₃ )[Hb]dB + (α^_λl - α^_A3 )[Hb0₂ ^B + AG_λl3.

29. The method of claim 26, wherein the step of determining a difference in attenuation of the light signal between the first wavelength and the second wavelength utilizes the equation: Axn = K _I ~ ^a x₂ )[Hb^B + (α^_λl - a_m' _θιλ2 )[Hb0₂ ]dB + AG_λ'_l2

and the step of determining a difference in attenuation of the light signal between the first wavelength and the third wavelength utilizes the equation: xn = ( - ' x )[Hb dB + { _m' _o%λl - α^_A3 )[Hbϋ₂ }ϊB + ΔG_A13.

30. A method for calibrating a near infrared spectrophotometric sensor for use in determining the blood oxygen saturation level within a subject's tissue, said method comprising the steps of: transmitting a light signal into the subject's tissue at a predetermined first intensity, wherein the transmitted light signal includes a first wavelength, a second wavelength, and a third wavelength; sensing a second intensity of the light signal along the first, second, and third wavelengths after the light signal travels through the subject; determining an attenuation of the light signal for each of the first, second, and third wavelengths using the predetermined first intensity and the sensed second intensity of the first, second, and third wavelengths; determining a difference in attenuation of the light signal between the first wavelength and the second wavelength, and between the first wavelength and the third wavelength; determining a first calibration constant and a second calibration constant using empirical data developed from the subject at or about the same time as when the sensing occurs; and calibrating the sensor using the first calibration constant and the second calibration constant.

31. The method of claim 30, wherein the empirical data is collected by discretely sampling a venous blood source and an arterial blood source from the subject.

32. The method of claim 30, wherein the empirical data is collected by continuously monitoring a venous blood source and an arterial blood source from the subject.

33. The method of claim 30, wherein the sensor is calibrated using equation: SmvO₂ = Kv*SvO₂ + Ka^*SaO₂.

34. The method of claim 33, wherein the step of determining a difference in attenuation of the light signal between the first wavelength and the second wavelength utilizes the equation: i_n = log[(/_A1 / Io_λl ) * /_A2 / Io_λ2 )] = Aa_cλX2cdB + AG_λl2 and the step of determining a difference in attenuation of the light signal between the first wavelength and the third wavelength utilizes the equation: as = logffci / ^Ioxχ ) * (⁷A₃ / Io_λ3 )] = Aa_cλl3cdB + ΔG '_AA13 -

35. The method of claim 33, wherein the step of determining a difference in attenuation of the light signal between the first wavelength and the second wavelength utilizes the equation:

Mm = (μ_m' _λx - a_H'_bλ2 )[Hb B + (α^_A1 - a_H'_bθ2λ2 )[Hb0₂ ]dB + ΔG_A12 and the step of determining a difference in attenuation of the light signal between the first wavelength and the third wavelength utilizes the equation:

Mm = μm' x ~ ^am' x₃ )[Hb^B + {a_H' _b0iXX - α^_A3 )[Hb^Q ₂ ]dB + ΔG 'A13 -

36. The method of claim 33, wherein the step of determining a difference in attenuation of the light signal between the first wavelength and the second wavelength utilizes the equation:

Mm = (μm' , ~ am' x₂ )[Hb lB + { _m' _θιλX - a_m' _θϊλ2 )[Hb0₂ ]dB + ΔG_A12 and the step of determining a difference in attenuation of the light signal between the first wavelength and the third wavelength utilizes the equation:

Miu = ( mx_ι ~ ^a x₃ )[Hb^B + (α^ _A1 - a_H'_bθ2λ3 )[Hb0₂ Jffl + ΔG '_Aλ₁l3 -

37. A method for calibrating a NIRS sensor, said method comprising the steps of: transmitting a light signal from a calibrated NIRS sensor into a reference sample at a predetermined first intensity, wherein the transmitted light signal includes a first wavelength, a second wavelength, and a third wavelength; sensing a second intensity of the light signal with the calibrated NIRS sensor along the first, second, and third wavelengths after the light signal travels through the reference sample; determining a first attenuation of the light signal for each of the first, second, and third wavelengths using the predetermined first intensity and the second intensity of the light signal sensed with the calibrated NIRS sensor; transmitting a light signal from an uncalibrated second NIRS sensor into the reference sample at the predetermined first intensity, wherein the transmitted light signal includes a first wavelength, a second wavelength, and a third wavelength; sensing a second intensity of the light signal with the uncalibrated second NIRS sensor along the first, second, and third wavelengths after the light signal travels through the subject; determining a second attenuation of the light signal for each of the first, second, and third wavelengths using the predetermined first intensity and the second intensity of the first, second, and third wavelengths sensed with the uncalibrated second NIRS sensor; adjusting the uncalibrated second NIRS sensor so that the second attenuation substantially agrees with the first attenuation.

38. A method for determining a blood oxygen saturation level within a subject's tissue attributable to venous blood, said method comprising the steps of: providing a NIRS sensor for sensing the subject's tissue; determining a blood oxygen saturation level with the subject's tissue using the NIRS sensor: determining a blood oxygen saturation level attributable to arterial blood within the subject's tissue using a pulse oximetry technique; and determining a blood oxygen saturation level attributable to venous blood within the subject's tissue using the equation:

„ _Λ CrS0₂ - (Ka * Sp0₂)

Sv0₂ = - — 1—2 .

² Kv

39. The method of claim 38, wherein the blood oxygen saturation level with the subject's tissue is determined using the following steps: transmitting a light signal into the subject's tissue at a predetermined first intensity, wherein the transmitted light signal includes a first wavelength, a second wavelength, and a third wavelength; sensing a second intensity of the light signal along the first, second, and third wavelengths after the light signal travels through the subject; determining an attenuation of the light signal for each of the first, second, and third wavelengths using the predetermined first intensity and the sensed second intensity of the first, second, and third wavelengths; determining a difference in attenuation of the light signal between the first wavelength and the second wavelength, and between the first wavelength and the third wavelength; determining a first calibration constant and a second calibration constant using empirical data developed from the subject at or about the same time as when the sensing occurs; determining the blood oxygen saturation level within the subject's tissue using the difference in attenuation between the first wavelength and the second wavelength, and the difference in attenuation between the first wavelength and the third wavelength.

40. A method for non-invasively determining a concentration of oxyhemoglobin and a concentration of deoxyhemoglobin within a subject's tissue using a near infrared spectrophotometric sensor, said method comprising the steps of:

(a) determining a blood oxygen saturation level with the subject's tissue by transmitting a light signal into the subject's tissue from a NIRS sensor at a predetermined first intensity, wherein the transmitted light signal includes a first wavelength, a second wavelength, and a third wavelength; sensing a second intensity of the light signal along the first, second, and third wavelengths after the light signal travels through the subject using the sensor; wherein the sensor is calibrated using empirical data that relates to the subject's tissue that is sensed by the sensor to account for light signal attenuation resulting from light signal scattering within the subject's tissue; determining an attenuation of the light signal for each of the first, second, and third wavelengths using the predetermined first intensity and the sensed second intensity of the first, second, and third wavelengths; determining a difference in attenuation of the light signal between the first wavelength and the second wavelength, and between the first wavelength and the third wavelength; determining a first calibration constant and a second calibration constant using empirical data developed from the subject at or about the same time as when the sensing occurs; determining the blood oxygen saturation level within the subject's tissue using the difference in attenuation between the first wavelength and the second wavelength, and the difference in attenuation between the first wavelength and the third wavelength, and the first calibration constant and the second calibration constant;

(b) determining a photon pathlength d*B; and

(c) determining the concentration of oxyhemoglobin and the concentration of deoxyhemoglobin within the subject's tissue using the first and second calibration constants.

41. The method of claim 40, wherein the concentration of oxyhemoglobin and the concentration of deoxyhemoglobin within the subject's tissue are determined using the equation:

where Ψ _b02 represents the first calibration constant, Ψ_Hb represents the second calibration constant, A_Hb02 represents a difference in attenuation of light signal attributable to oxyhemoglobin, and A_Hb represents a difference in attenuation of light signal attributable to deoxyhemoglobin.

42. A method for non-invasively determining a concentration of oxyhemoglobin and a concentration of deoxyhemoglobin within a subject's tissue at an initial time t1 and a subsequent time t2 using a near infrared spectrophotometric sensor, said method comprising the steps of:

(b) determining a photon pathlength d*B;

(c) determining the concentration of oxyhemoglobin and the concentration of deoxyhemoglobin within the subject's tissue at the initial time t1 using the equation:

where Ψ_Hb02 represents the first calibration constant, Ψ_Hb represents the second calibration constant, A_Hb02 represents a difference in attenuation of light signal attributable to oxyhemoglobin, and A_Hb represents a difference in attenuation of light signal attributable to deoxyhemoglobin; and

(d) determining a change in the concentration of oxyhemoglobin and a change in the concentration of deoxyhemoglobin from the initial time t1 to a subsequent second time t2 are determined using the equation:

-

;and (e) determining the concentration of oxyhemoglobin and the concentration of deoxyhemoglobin within the subject's tissue at the subsequent time t2 using the equations:

[Hb]_t2 = AHb(t₂) + [Hb] and [Hb0₂]_t = AHb0₂(t₂) + [Hb0₂] .

43. A method for non-invasively determining a blood oxygen saturation level within a subject's tissue using a near infrared spectrophotometric sensor, said method comprising the steps of: transmitting a light signal into the subject's tissue at a predetermined first intensity using the sensor; sensing a second intensity of the light signal along three or more selectively chosen wavelengths after the light signal travels through the subject using the sensor; wherein the sensor is calibrated using empirical data that relates to the subject's tissue that is sensed by the sensor to account for light signal attenuation resulting from light signal scattering within the subject's tissue; determining an attenuation of the light signal for at least "n" number of the selectively chosen wavelengths using the predetermined first intensity and the sensed second intensity of the selectively chosen wavelengths, where "n" is an integer equal to or greater than three; determining a difference in attenuation of the light signal between a first wavelength and each of "n" number of the selectively chosen wavelengths; determining the blood oxygen saturation level within the subject's tissue using the difference in attenuation between the first wavelength and each of the "n" number of other selectively chosen wavelengths.

44. A method for determining a blood oxygen saturation level within a subject's tissue using a near infrared spectrophotometric sensor attached to the skin of the subject, said method comprising the steps of: transmitting a light signal into the subject's tissue at a predetermined first intensity; sensing a second intensity of the light signal along three or more selectively chosen wavelengths after the light signal travels through the subject; determining an attenuation of the light signal for at least "n" number of the selectively chosen wavelengths using the predetermined first intensity and the sensed second intensity of the selectively chosen wavelengths, where "n" is an integer equal to or greater than three; determining a difference in attenuation of the light signal between a first wavelength and each of "n" number of the selectively chosen wavelengths; determining a first calibration constant and a second calibration constant using empirical data developed from the subject at or about the same time as when the sensing occurs; determining the blood oxygen saturation level within the subject's tissue using the difference in attenuation between the first wavelength and each of "n" number of the selectively chosen wavelengths, and the first calibration constant and the second calibration constant.